<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style>
    * {
        margin: 0;
        padding: 0;
    }

    body {
        position: relative;
    }

    .wrap {
        width: 100px;
        height: 100px;
        border-radius: 50%;
        position: absolute;
    }
</style>
<body>
</body>
<script>
    var width = window.innerWidth;
    var height = window.innerHeight;
    for (var i = 0; i < 100; i++) {
        var span = document.createElement("span");
        span.className = "wrap";
        var numw = Math.random() * width - 100;
        var numh = Math.random() * height - 100;
        span.style.top = numh + "px";
        span.style.left = numw + "px";
        span.style.backgroundColor = f();
        document.body.appendChild(span);
    }

    function f() {
        color = "#";
        for (var i = 0; i < 6; i++) {
            color += (Math.random() * 16 | 0).toString(16);
        }
        return color;
    }
</script>
</html>